The Discovery and Nature of the Optical Transient Css100217:102913+404220∗,†

THE DISCOVERY AND NATURE OF THE OPTICAL TRANSIENT CSS100217:102913+404220∗,†

A. J. Drake1, S. G. Djorgovski1, A. Mahabal1, J. Anderson2,R.Roy3, V. Mohan4, S. Ravindranath4, D. Frail5, S. Gezari6, James D. Neill1,L.C.Ho7, J. L. Prieto7, D. Thompson8, J. Thorstensen9, M. Wagner8, R. Kowalski10, J. Chiang11, J. E. Grove12, F. K. Schinzel13,D.L.Wood12, L. Carrasco14, E. Recillas14, L. Kewley15, K. N. Archana16,17, Aritra Basu17, Yogesh Wadadekar17, Brijesh Kumar3, A. D. Myers18, E. S. Phinney1, R. Williams1, M. J. Graham1, M. Catelan19, E. Beshore10, S. Larson10, and E. Christensen20 1 California Institute of Technology, 1200 E. California Blvd., CA 91225, USA 2 STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA 3 Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263129, Uttarakhand, India 4 IUCAA, Postbag 4, Ganeshkhind, Pune 411007, India 5 NRAO, Campus Building 65, 949 North Cherry Avenue, Tucson, AZ 85721-0655, USA 6 Department of Physics & Astronomy, Johns Hopkins University, 366 Bloomberg Center, 3400 N. Charles Street, Baltimore, MD 21218, USA 7 Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA 8 LBT, University of Arizona, 933 N. Cherry Ave, Room 552, Tucson, AZ 85721, USA 9 Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755-3528, USA 10 Department of Planetary Sciences, Lunar and Planetary Laboratory, The University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA 11 W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, Stanford University, Standford, CA 94305, USA 12 Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA 13 Max-Planck-Institut fur¨ Radioastronomie, Auf dem Hugel¨ 69, 53121 Bonn, Germany 14 INAOE, Tonantzintla, Puebla, Mexico 15 IFA, 640 North A’ohoku Place, #209, Hilo, HI 96720-2700, USA 16 School of Computer Sciences, Mahatma Gandhi University, Kottayam 686560, India 17 National Centre for Radio Astrophysics, TIFR, Post Bag 3, Ganeshkhind, Pune 411007, India 18 Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 19 Departamento de Astronom´ıa y Astrof´ısica, Pontiﬁcia Universidad Catolica´ de Chile, Av. Vicuna˜ Mackena 4860, 782-0436 Macul, Santiago, Chile 20 Gemini Observatory, Casilla 603, La Serena, CL, Chile Received 2010 December 17; accepted 2011 April 26; published 2011 June 22

ABSTRACT We report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations showed that this transient was coincident with a galaxy at redshift z = 0.147 and reached an apparent magnitude of V ∼ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be MV =−22.7 approximately 45 days after maximum light. Over a period of 287 rest-frame days, this event had an integrated bolometric luminosity of 1.3 × 1052 erg based on time-averaged bolometric corrections of ∼15 from V- and R-band observations. Analysis of the pre-outburst Sloan Digital Sky Survey (SDSS) spectrum of the source shows features consistent with a narrow-line Seyfert 1 galaxy. High-resolution Hubble Space Telescope and Keck follow-up observations show that the event occurred within 150 pc of the nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival Catalina Sky Survey NEO search and SDSS data to investigate the three most likely sources of such an event: (1) an extremely luminous supernova, (2) the tidal disruption of a star by the massive nuclear black hole, and (3) variability of the central active galactic nucleus (AGN). We find that CSS100217 was likely an extremely luminous Type IIn supernova and occurred within the range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity. Key words: galaxies: active – galaxies: nuclei – galaxies: stellar content – supernovae: general Online-only material: color figures

1. INTRODUCTION vational frontier has fueled the advent of the new generation of digital synoptic sky surveys, which cover the sky many times, Exploration of the time domain is now one of the most rapidly as well as the necessity of using robotic telescopes to respond growing and exciting areas of astrophysics. This vibrant obser- rapidly to transient events (Paczynski 2000). However, the discovery of transient astronomical events is by no means new to ∗ Some of the data presented herein were obtained at the W. M. Keck astronomy with phenomena such as supernovae being discov- Observatory, which is operated as a scientiﬁc partnership among the California ered and documented for centuries (Zhao et al. 2006). Transient Institute of Technology, the University of California, and the National events themselves have been observed on timescales from sec- Aeronautics and Space Administration. The Observatory was made possible by the generous ﬁnancial support of the W. M. Keck Foundation. onds; e.g., gamma-ray bursts (GRBs; Klebesadel et al. 1973), † Based on observations made with the NASA/ESA Hubble Space Telescope, to years; e.g., supernovae (Rest et al. 2011) and active galactic obtained at the Space Telescope Science Institute, which is operated by the nuclei (AGNs; Ulrich et al. 1997). Predictions of new types of Association of Universities for Research in Astronomy, Inc., under NASA observable astrophysical phenomena continue to be theorized contract NAS 5-26555. These observations are associated with program 12117.

1 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al. and made detectable by advancing technology. For example, services such as DataScope.22 Events that remain a poor match the possibility that Massive Compact Halo Objects could be for known types phenomena are of particular interest and are observable due to gravitational microlensing was theorized by the most closely scrutinized. Paczynski (1986) and soon proven by Alcock et al. (2003) and Aubourg et al. (1993). Such discoveries set the stage for larger 2. THE DISCOVERY OF CSS100217 time domain surveys with broader goals. A systematic exploration of the observable parameter space On 2010 February 17 we discovered the transient event in the time domain is very likely to lead to many new dis- CSS100217 during the course of the CRTS. This event was coveries (e.g., Djorgovski et al. 2001a, 2001b, and references flagged as unusual at discovery as the object had a past spectrum therein). For example, many types of transient events have been from the SDSS (Abazajian et al. 2009) that resembled a Seyfert, theoretically predicted, yet remain to be convincingly observed. yet the outburst was uncharacteristically rapid and large for an Among these are GRB orphan afterglows (Nakar et al. 2002) AGN. Additionally, there was no detection in archival FIRST and the electromagnetic counterparts to gravitational-wave in- and NRAO VLA Sky Survey (NVSS) radio data covering the spiral events, where close binaries coalesce to release a surge object’s location. The lack of any radio source suggests that the of gravitational radiation (Abadie et al. 2010). Nevertheless, the variability was not being powered by a jet, as seen with optically recent detections of rare types of transients, including candidate variable blazars. Given the unexpected nature of the event, we pair-instability supernovae (Gal-Yam et al. 2010), tidal disrup- immediately scheduled photometric and spectroscopic follow- tion events (TDEs; Gezari et al. 2009a; van Velzen et al. 2010), up of the event. and supernova shock breakouts (Soderberg et al. 2008; Schaw- inskietal.2008) show the promise of current and future large 3. MULTI-WAVELENGTH OBSERVATIONS transient surveys. Following the discovery of the clearly energetic event we Recent optical surveys searching for transient phenomena undertook follow-up observations in X-ray, UV, optical, near- include ROTSE (Akerlof et al. 2000), Sloan Digital Sky Survey IR, and radio wavelengths as well as a targeted archival gamma- (SDSS; Sesar et al. 2007), Palomar Quest (PQ; Djorgovski et al. ray search. In Table 1, we present the sequence and nature of the 2008), the Catalina Real-time Transient Survey (CRTS; Drake follow-up observations we obtained. In the following section et al. 2009), Palomar Transient Factory (PTF; Rau et al. 2009), we will discuss the details of each set of observations as well as and the Panoramic Survey Telescope & Rapid Response System historical data for the source galaxy. We will then combine the (PanSTARRS; Hodapp et al. 2004). In the near future additional data to interpret the nature of the event in relation to known types surveys such as SkyMapper (Keller et al. 2007) and the Large of transients and make concluding remarks about the source. Synoptic Survey Telescope (LSST; Ivezic et al. 2008) will begin operation. At radio wavelengths work is being undertaken by 3.1. UV, Optical, and Near-IR Photometry the low-frequency array (Rottgering 2003), the Allen Telescope Array (Croft et al. 2009) and will soon begin with the Australian Following CSS100217’s discovery, unfiltered observations Square Kilometre Array Pathfinder (Johnston et al. 2007). At continued as part of the CSS survey. All CSS photometry high energies ongoing satellite searches for transients include is routinely transformed to V-magnitudes by using between Fermi 10 and 100 G-type dwarf calibration stars measured in each the Large Area Telescope (LAT; Atwood et al. 2009), 2 Swift Galaxy Evolution Explorer 8deg image. These calibration stars are pre-selected using (Barthelmy et al. 2005), and 23 (GALEX; Martin et al. 2005), Rossi X-ray Timing Explorer Two Micron All Sky Survey (2MASS) near-IR data. The (Jahoda et al. 1996), and the Monitor of All-sky X-ray Image magnitudes for each calibration star are transformed to V (Ueno et al. 2008). following Bessell & Brett (1988), and the zero point for each As the temporal and spatial coverage of transient surveys in- field is derived. The scatter in the V magnitude for the calibration creases, there are prospects for the discovery of rare events on stars is typically <0.05 mag (Larson et al. 2003). To improve short timescales. To this end, the new generation of transient sur- the quality of the photometry for CSS100217 we created a veys are increasingly working on real-time analysis and detec- high signal-to-noise ratio (S/N) template image and carried tion. In late 2007, the CRTS (Drake et al. 2009) simultaneously out image subtraction (Tomaney & Crotts 1996)onallthe began real-time analysis and notification of events in images CSS images. This process reduces the photometric dependence taken by the Catalina Sky Survey NEO search (CSS; Larson on external calibrators, but can introduce uncertainty due to et al. 2003). The CRTS transient survey currently analyzes data the subtraction process. Based on our analysis the zero-point from three telescopes operated by CSS. These telescopes cover uncertainty is approximately 0.1 mag. ∼1800 deg2 on the sky per night to a depth ranging from V = 19 In Figure 1, we present the CSS light curve of the transient to 21.5. New objects are automatically flagged in real time and plus host galaxy flux determined from the template image. In filtered to isolate genuine optical transients from artifacts and order to determine the brightness of the transient we carried out other noise sources. In order to maximize discovery potential image subtraction (Tomaney & Crotts 1996) on the CSS data all CRTS transients are immediately distributed publicly as VO- using a combined high signal-to-noise template image produced Events.21 For each of the few dozen transient events detected per from observations taken before the outburst. The flux of the night a portfolio of historical observational information is ex- transient source is thus in units of the template image flux. We tracted from past surveys from radio to gamma-ray wavelengths. determine the brightness of the transient by calibrating the flux This information allows most events to be classified into a few using the magnitudes of stars measured in the template image. broad types of phenomena including supernovae, blazars, and CSS photometry is available in Table 2 . SDSS photometric cataclysmic variable outbursts. Objects which are of uncertain observations taken in late 2002 and early 2003 exhibit r and nature are examined in greater detail using Virtual Observatory i magnitudes of 17.6 and 17.3, respectively, and is given in

22 http://heasarc.gsfc.nasa.gov/cgi-bin/vo/datascope/init.pl 21 http://crts.caltech.edu/ and SkyAlert, http://www.skyalert.org/. 23 http://www.ipac.caltech.edu/2mass/releases/allsky/doc/explsup.html

2 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Table 1 Observation Sequence

Telescope + Instrument Passbands Observation Date (YYYY-MM-DD) Photometry Filters SDSS u, g, r, i, z 2002-12-31 × 2, 2003-03-26 CSS + 4K CDD V (unﬁltered) 2003–2010 ARIES 1 m U, B, V, R, I 2010-04+ Palomar 1.5 m g, r, i, z 2010-03-02, 2010-03-11, 2010-06-22, 2010-06-23 SWIFT + UVOT UVW1,UVM2,UVW2,U,B,V 2010-04-06, 2010-04-25, 2010-05-09, 2010-05-23 GALEX NUV, FUV 2004-01-24, 2010-01-29, 2010-04-17, 2010-04-29 HST + WFC3 F390W, F555W, F763M 2010-05-31 Keck + AO NIR 2010-06-02 LBT + LUCIFER Ks 2010-05-02 GHO 2.1 m +CANICA J, H, Ks 2010-04+ Spectra Wavelength Range (Å) SDSS + MOS 3800–9100 2002-12-29 IGO + IFOSC 4000–8600 2010-02-18, 2010-04-04, 2010-04-23 P200 + DBSP 3500–9100 2010-03-15, 2010-11-09 APO + DIS 3500–9600 2010-04-09 GALEX + NUV grism 1900–2800 2010-04-17, 2010-04-29 MDM 2.4 m + Modspec 4200–7560 2010-05-04 Keck-I + LRIS 3100–10100 2010-05-18 Radio Central Frequency EVLA 4.5 GHz + 7.9 GHz 2010-04-29, 2010-05-14, 2010-06-01 GMRT 608 mHz 2010-05-23 X-ray and γ -ray Energy Swift +XRT 0.2–10 keV 2010-04-06 Fermi +LAT 20 MeV–300 GeV 2009-11-20 to 2010-07-04

Figure 1. VCSS light curves of CSS100217 taken with the 0.7 m Catalina Schmidt telescope with respect to Modiﬁed Julian Date and maximum light. Left: the full CSS light curve covering host and event. Right: the event light curve after subtracting the galaxy ﬂux. The dates on which the IGO, P200, APO, MDM, and Keck follow-up spectra were observed are marked with arrows.

Table 3. Additional measurements in the USNO-B1.0 catalog Sampurnanand telescope in Nainital, India. Data were reduced (Monet et al. 2003) with epoch 1977.1, list the object with by performing point-spread function (PSF) photometry using magnitudes B2 = 17.6, R2 = 17.4, I = 17.2. This suggests DAOPHOT (Stetson 1987). The photometry was calibrated that the object was relative stable on a very long timescale. using Landolt’s (1992) standard stars’ fields PG1047+003 and Optical follow-up was taken soon after discovery with the PG1323-085. Ten bright isolated stars within the field of Palomar 1.5 m telescope (P60) in gunn g,r,i,z filters. Data were CSS100217 were used as local standards and to derive the also taken in Johnson U, B, V and Cousins R, I using the 1 m zero points for the images of the transient. To supplement the

3 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Table 2 Table 2 CSS Photometry of CSS100217 (Continued)

MJD Phase VCSS MJD Phase VCSS 54884.246 −365.75 16.82 ± 0.08 55513.425 263.43 16.66 ± 0.10 54884.251 −365.75 16.87 ± 0.06 55513.433 263.43 16.65 ± 0.10 54884.263 −365.74 16.76 ± 0.18 55513.441 263.44 16.66 ± 0.10 54892.171 −357.83 16.92 ± 0.07 54892.205 −357.80 16.95 ± 0.04 Notes. The phase of the event is taken relative to the adopted maximum bright 54911.194 −338.81 16.87 ± 0.05 at MJD 55250. The photometry includes the flux measured for the host galaxy 54911.211 −338.79 16.88 ± 0.05 in the difference image template. 54922.299 −327.70 16.91 ± 0.05 54922.323 −327.68 16.82 ± 0.07 54945.252 −304.75 16.97 ± 0.04 optical data and constrain the possibility that this was a TDE, 54945.272 −304.73 16.95 ± 0.05 we requested Swift Target of Opportunity time to observe the 55160.445 −89.55 16.78 ± 0.16 transient at X-ray and UV wavelengths. These observations 55160.453 −89.55 16.82 ± 0.08 clearly showed that the object was bright at UV wavelengths 55160.461 −89.54 16.82 ± 0.09 and a source was detected in the X-rays. 55160.469 −89.53 16.75 ± 0.17 In Figure 2, we present the photometry of the CSS100217 55183.410 −66.59 16.32 ± 0.09 during the early decline phase in six Swift filters uvw1, uvw2, 55183.414 −66.59 16.32 ± 0.07 − ± uvm2, u, b, v and ground-based U, B, V, R, I.TheB and b, and 55183.419 66.58 16.39 0.11 V and v magnitudes are in excellent agreement. However, the 55183.424 −66.58 16.32 ± 0.07 55211.379 −38.62 15.80 ± 0.03 magnitudes vary between U and u as the Bessell filter has central 55211.386 −38.61 15.83 ± 0.03 wavelength 3663 Å and effective width 650 Å while the Swift U 55211.394 −38.61 15.80 ± 0.02 filter is bluer and broader (central wavelength 3465 Å, width 55211.401 −38.60 15.83 ± 0.03 785 Å). Variations in the spectral energy distribution (SED) 55244.284 −5.72 15.74 ± 0.02 between these two filters likely causes most of the observed 55244.291 −5.71 15.77 ± 0.02 difference. Li et al. (2006) gives the transformation from Swift 55244.303 −5.70 15.71 ± 0.02 filters to standard Johnson filters. For b and v the difference 55272.198 22.20 15.77 ± 0.02 from standard filters is of order 0.02 mag. However, for objects 55272.203 22.20 15.77 ± 0.02 − ± such as CSS100217 with u v<0 the difference noted by 55272.208 22.21 15.75 0.02 Li et al. (2006) is significant and the transformation to standard 55272.213 22.21 15.80 ± 0.02 55297.123 47.12 15.82 ± 0.02 magnitudes is very poorly constrained. Therefore, we have not 55297.131 47.13 15.82 ± 0.03 attempted to transform the Swift magnitudes to the standard 55297.138 47.14 15.84 ± 0.02 system. 55297.146 47.15 15.83 ± 0.03 From the CSS photometry we find that the peak luminosity 55323.210 73.21 15.97 ± 0.03 occurred on 2010 February 23 (MJD 55250). Hereafter we 55323.216 73.22 16.00 ± 0.03 denote this date as Tp. We fit the decline rate in each photometric 55323.221 73.22 15.98 ± 0.03 filter over the range of dates from Tp +42toTp + 112. The 55323.227 73.23 16.00 ± 0.04 observed decline of CSS100217 plus the host galaxy is close to 55328.254 78.25 16.01 ± 0.04 −1 ± linear and varies between bands from 0.012 mag day in uvm2 55328.261 78.26 16.06 0.04 to 0.0064 mag day−1 in I. The rapid decline at bluer wavelengths 55328.267 78.27 16.00 ± 0.04 55337.188 87.19 16.03 ± 0.05 is consistent with an outburst that is cooling with time. The rate 55337.196 87.20 16.04 ± 0.04 of decline is consistent with the range observed for Type IIn 55337.203 87.20 16.04 ± 0.04 supernovae (Trundle et al. 2009). However, we note that the true 55337.211 87.21 16.03 ± 0.04 decline rate of the transient is much greater as the photometry 55354.159 104.16 16.13 ± 0.04 includes both the galaxy and transient light. In order to determine 55354.163 104.16 16.14 ± 0.04 the true decline rate for CSS100217 we transformed the SDSS 55354.168 104.17 16.14 ± 0.04 photometry of the host galaxy to Bessell magnitudes. As the 55354.173 104.17 16.18 ± 0.05 SED of the host galaxy varies from that expected from stars, we 55355.154 105.15 16.13 ± 0.05 ± preformed the transformation in two ways that follow Jester et al. 55355.159 105.16 16.18 0.05 (2005). First, we used the transformations derived for QSOs 55355.164 105.16 16.19 ± 0.05 ± with redshift z<2.1 and second for stars (R − Ic < 1.15 and 55355.169 105.17 16.14 0.05 − 55362.163 112.16 16.22 ± 0.05 U B<0). Using the relation for stars we obtain host galaxy 55362.168 112.17 16.18 ± 0.05 flux U = 17.55, B = 18.17, V = 17.77, Rc = 17.25, and 55362.172 112.17 16.21 ± 0.05 Ic = 16.71. With the QSO transformation we obtain U = 17.51, 55362.177 112.18 16.18 ± 0.05 B = 18.07, V = 17.77, Rc = 17.38, and Ic = 16.88. We 55381.168 131.17 16.31 ± 0.08 note that the difference in magnitudes using the two different ± 55381.169 131.17 16.29 0.07 methods is maximum for Ic (0.17 mag) and for V produces the 55381.170 131.17 16.30 ± 0.07 same result. Hereafter, we adopt the transformation for stars but 55381.171 131.17 16.28 ± 0.07 ± note that the use of the other set of equations will produce a 55497.470 247.47 16.70 0.15 small constant shift. 55497.476 247.48 16.64 ± 0.10 55497.482 247.48 16.66 ± 0.10 After subtracting the host flux contribution from each mea- 55497.487 247.49 16.61 ± 0.10 surement we combined uncertainties in photometry in quadra- 55513.417 263.42 16.67 ± 0.10 ture, with the conversion errors noted by Jester et al. (2005),

4 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Table 3 Filtered Photometry of CSS100217 and Host Galaxy

MJD Phase Band1 Band2 Band3 Band4 Band5 Band6 SDSS ugr i z 52639.380 −2610.62 18.20 ± 0.01 17.84 ± 0.01 17.63 ± 0.01 17.29 ± 0.01 17.34 ± 0.01 52639.470 −2610.53 18.17 ± 0.01 17.84 ± 0.01 17.64 ± 0.01 17.35 ± 0.01 17.36 ± 0.01 52724.240 −2525.76 18.24 ± 0.01 17.90 ± 0.01 17.70 ± 0.01 17.38 ± 0.01 17.45 ± 0.01 Pal. 1.5 m griz 55257.248 7.25 16.43 ± 0.22 16.27 ± 0.17 16.04 ± 0.08 16.18 ± 0.12 55257.381 7.38 ... 16.48 ± 0.08 16.10 ± 0.12 16.07 ± 0.19 55266.508 16.51 16.44 ± 0.21 16.26 ± 0.16 16.05 ± 0.19 16.30 ± 0.21 55369.171 119.17 17.58 ± 0.19 17.31 ± 0.19 16.80 ± 0.23 16.68 ± 0.21 55370.174 120.17 17.73 ± 0.15 17.31 ± 0.18 16.75 ± 0.21 16.68 ± 0.30 ARIES UBVR I 55297.771 47.77 16.45 ± 0.05 16.94 ± 0.02 16.56 ± 0.01 16.38 ± 0.02 15.92 ± 0.02 55298.799 48.80 16.52 ± 0.05 16.95 ± 0.02 16.53 ± 0.02 16.40 ± 0.04 15.87 ± 0.03 55299.637 49.64 16.41 ± 0.06 16.93 ± 0.05 16.52 ± 0.02 16.36 ± 0.02 ... 55300.643 50.64 16.59 ± 0.06 ...... 16.40 ± 0.02 15.91 ± 0.03 55310.656 60.66 16.55 ± 0.03 ...... 16.47 ± 0.02 15.90 ± 0.02 55311.613 61.61 16.61 ± 0.05 17.11 ± 0.02 ... 16.49 ± 0.02 ± 0.02 55312.632 62.63 16.72 ± 0.05 17.11 ± 0.04 16.67 ± 0.03 16.59 ± 0.03 15.94 ± 0.03 55316.604 66.60 16.73 ± 0.05 17.20 ± 0.01 16.72 ± 0.01 16.58 ± 0.02 15.97 ± 0.02 55320.610 70.61 16.68 ± 0.14 ...... 16.61 ± 0.03 16.03 ± 0.02 55323.646 73.65 16.85 ± 0.04 17.30 ± 0.01 16.83 ± 0.02 16.71 ± 0.02 16.08 ± 0.02 55324.601 74.60 16.89 ± 0.04 17.30 ± 0.03 16.83 ± 0.01 16.69 ± 0.03 16.09 ± 0.03 55329.625 79.63 ...... 16.72 ± 0.03 ... 55334.624 84.62 16.92 ± 0.03 17.41 ± 0.03 16.97 ± 0.02 16.85 ± 0.03 16.18 ± 0.05 55346.611 96.61 ... 17.60 ± 0.02 17.10 ± 0.02 16.99 ± 0.03 ± 0.03 55349.629 99.63 17.28 ± 0.07 17.65 ± 0.05 17.14 ± 0.03 17.01 ± 0.04 16.29 ± 0.03 55356.654 106.65 17.19 ± 0.07 17.70 ± 0.03 17.21 ± 0.02 17.14 ± 0.03 16.41 ± 0.04 55358.620 108.62 17.25 ± 0.05 17.82 ± 0.03 17.21 ± 0.03 17.23 ± 0.05 16.49 ± 0.04 55362.622 112.62 17.14 ± 0.04 17.77 ± 0.02 17.23 ± 0.02 ... 16.49 ± 0.04 55364.619 114.62 ... 17.80 ± 0.06 17.19 ± 0.06 ... 16.49 ± 0.04 55495.983 245.98 ... 19.49 ± 0.20 18.61 ± 0.08 19.15 ± 0.20 17.64 ± 0.10 55499.932 249.93 18.97 ± 0.20 19.18 ± 0.09 18.74 ± 0.09 19.43 ± 0.25 17.77 ± 0.09 55509.979 259.98 19.28 ± 0.24 19.41 ± 0.09 18.58 ± 0.06 19.20 ± 0.19 17.99 ± 0.13 55527.971 277.97 18.84 ± 0.19 ... 18.97 ± 0.10 20.58 ± 0.77 ... 55533.979 283.98 19.10 ± 0.28 19.97 ± 0.18 19.10 ± 0.10 20.68 ± 0.86 17.96 ± 0.29 55562.971 312.97 19.41 ± 0.30 21.11 ± 0.49 19.41 ± 0.13 ... 18.24 ± 0.30 Swift UVW1 UVM2 UVW2 U B V 55292.580 42.58 16.10 ± 0.03 16.09 ± 0.02 16.27 ± 0.02 15.76 ± 0.03 16.55 ± 0.03 16.18 ± 0.04 55311.620 61.62 16.19 ± 0.10 16.44 ± 0.07 16.43 ± 0.03 15.90 ± 0.03 16.80 ± 0.03 16.32 ± 0.04 55325.940 75.94 16.39 ± 0.03 16.46 ± 0.03 16.64 ± 0.03 16.07 ± 0.03 16.84 ± 0.03 16.35 ± 0.04 55339.120 89.12 16.78 ± 0.06 16.66 ± 0.08 16.71 ± 0.02 16.17 ± 0.06 16.98 ± 0.03 16.56 ± 0.04

Near-IR JHKs 55312.176 62.18 15.06 ± 0.11 14.19 ± 0.13 13.54 ± 0.09 55313.311 63.31 15.23 ± 0.12 14.38 ± 0.14 13.44 ± 0.12 55333.230 83.23 15.03 ± 0.10 14.28 ± 0.12 13.17 ± 0.11 55336.209 86.21 15.22 ± 0.11 14.18 ± 0.13 13.03 ± 0.10 55338.175 88.18 14.99 ± 0.11 14.38 ± 0.14 13.69 ± 0.09 55340.162 90.16 15.59 ± 0.13 14.46 ± 0.12 13.74 ± 0.10 55342.155 92.16 15.41 ± 0.11 14.32 ± 0.12 13.51 ± 0.10 55361.191 111.19 15.15 ± 0.11 14.09 ± 0.12 13.17 ± 0.09 55369.147 119.15 15.52 ± 0.11 14.33 ± 0.12 13.72 ± 0.09

Notes. The phase of the event is taken relative to the adopted maximum bright at MJD 55250. SDS photometry is that of the host galaxy. Swift photometry measurements include the flux from the host galaxy. All other photometry have the host galaxy flux subtracted. then fit each with a simple linear decline over the time between that of SDSS, therefore, we subtract the SDSS host magnitudes. Tp + 42 and Tp + 112. The following declines rates were The host-subtracted-filtered optical photometry from Palomar found: ΔU = 0.0121(0.0017), ΔB = 0.0134(0.0018), 1.5 m and ARIES 1 m is presented in Table 3,asistheSwift ΔV = 0.0112(0.0026), ΔR = 0.0132(0.0021), ΔI = photometry for this event. 0.00985(0.0019). The slope derived from the CSS light curve Based on our filtered photometry, the CSS transformed V during this period was ΔVCSS = 0.0117(0.0022), and thus is magnitudes (VCSS) lie very close to R magnitudes. The decline very close to that observed in the filtered V photometry. For the rate is also very similar. The VCSS values are slightly brighter Palomar 1.5 m photometry in the gunn filter system matches than filtered V magnitudes (ΔV<0.2 mag). The departure of

5 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Figure 2. Multicolor evolution of CSS100217 near peak from the Swift space Figure 3. Near-IR light curves of CSS100217 in J, H,andKs bands. The short- telescope and ARIES 1 m. Swift UVOT observations are in uvw1, uvw2, uvm2, dashed line presents the observed Ks-band values, the long-dashed line connects u, v,andb bands. ARIES observations are in Johnson U, B, V and Cousins R H-band measurements, and the solid line J-band magnitudes. and I bands. (A color version of this ﬁgure is available in the online journal.) (A color version of this ﬁgure is available in the online journal.)

gunn measurements taken at this time we find Mg =−22.8, CSS photometry from Bessell V is due to the non-stellar SED of Mr =−23.0, Mi =−23.2, and Mz =−23.1, without the transient source combined with the galaxy. In particular, the K-correction. Similarly, for ARIES photometry, taken at Tp+40, strong Hα emission lies within the CSS transmission sensitivity we find MU =−22.9, MB =−22.3, and MI =−23.3, without as well as the overlap between R and I filter response (well K-correction. beyond V filter response). The peak brightness in V band is similar to that observed The foreground Galactic extinction for this event is AV = for the Type IIn supernovae SN 2008fz (Drake et al. 2010a, 0.046, based on Schlegel et al. (1998) reddening maps. We do MV =−22.3) and Type IIL supernova SN 2008es (MV = not see evidence for host galaxy extinction in the spectra of −22.2, Gezari et al. 2009b; MV =−22.3, Miller et al. event, suggesting this is a small effect. Thus, we only correct 2009). Another luminous Type IIn, SDWFS-MT-1 (aka SN for foreground extinction. Applying the V-band reddening 2007va) was detected in Spitzer data and was observed with correction and a K-correction of 0.15 mag, to account for the M[4.5] ∼−24.2in4.5 μm Spitzer/IRAC band (Kozlowski et al. effective rest-frame bandwidth, we find that the peak luminosity 2010). =− = −1 −1 was MVCSS 23.0. We adopt H0 72 km s mpc , Near-IR J, H, and Ks observations were carried out with ΩΛ = 0.73, and Ωm = 0.27 with the host galaxy’s redshift of CANICA, a NIR camera equipped with a 1024 × 1024 pixel z = 0.147. To derive K-correction in filtered photometry near Hawaii array, at the 2.1 m telescope of the Guillermo Haro peak we first subtracted the SDSS spectrum from the IUCAA Observatory located in Cananea, Sonora, Mexico. Data were Girawali Observatory (IGO) follow-up spectrum observed near reduced using standard procedures. As some observations were peak. Using the Palomar spectrum that was taken near maximum carried out on partially cloudy nights, differential photometry light (Tp+ 20 days), we derive the K-corrections in the V and R for the object and field stars was carried out. For the latter, we filters of −0.01 and 0.04, respectively. As the filter transmission adopted the photometric values listed in the 2MASS All-Sky range of our other filtered observations goes beyond that of Catalog of Point Sources (Skrutskie et al. 2006). In Figure 3,we this spectrum (3950–9050 Å), K-corrections are not derived for present the near-IR measurements and in Table 3 we include other filters. the host-subtracted near-IR photometry. The peak-observed The extinction and K-corrected peak magnitudes measured apparent brightness of CSS100217 in near-IR, after subtracting at Tp +40areMV =−22.7 ± 0.3 and MR =−22.8 ± 0.3. the host brightness using 2MASS magnitudes, is J ∼ 15.0, H ∼ Here, we have combined estimates for uncertainties in the true 14.1, and Ks ∼ 13.0. The corresponding absolute magnitudes =− =− =− color variation near peak (0.1), the photometric zero points are thus MJ 24.2, MH 25.1, and MKs 26.2 (again (0.15), the K-corrections (0.1), and the color transformations without K-correction). However, we note that there may be from SDSS filters (0.17). For the Palomar 1.5 m gunn pho- significant uncertainty in the near-IR brightness of the transient. tometry taken at Tp + 7, we use the IGO spectrum taken at Once the event has fully faded, it will be possible to determine Tp − 5 and find a K-correction of Kr =−0.16 and thus the host contribution more accurately and thus the event’s Mr =−22.8 ± (0.3). Here, there is no need to transform the peak brightness in each filter. Unlike the optical data there is filter system as the observations are not long after maximum. little evidence for a decline in luminosity. This effect may be However, the photometric uncertainties are larger. For the other attributed to the cooling of expanding material. The object is

6 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al. long timescales (Webb & Malkan 2000). In contrast optical variability of many magnitudes can be seen in blazars (Bauer et al. 2009) which have broad lines and are associated with strong radio sources and have smooth featureless continua during their outbursts. In this case the emission lines observed from the galaxy were relatively narrow and no radio detection has been made in past radio surveys by FIRST (1.4 GHz, Becker et al. 1995), WENSS (326 mHz, Rengelink et al. 1997), and NVSS (1.4 GHz, Condon et al. 1998) at the level of 1 mJy beam−1. The emission line features observed in the SDSS spectrum are very clearly asymmetric. In addition, the Balmer lines appear to have a broad component. We decomposed the SDSS spectrum to determine the nature of the SDSS source. For Hβ we see three clear significant components with narrow, medium, and broad velocity widths. There were clearly systematic offsets between the broad-, medium-, and narrow-line components. For [O iii] we found just narrow and medium width components that were consistent with the Hβ emission features. For the Hα region the line fitting process is more complex because of the presence of blended N ii emission lines. First, we fit the line complex with individual [N ii] 6548 Å and 6583 Å lines of fixed 1/3 flux ratio plus Hα emission lines considering just a narrow and broad Figure 4. Archival SDSS DR7 spectrum of the host galaxy to CSS100217 component. The fit result was quite poor as the Hα has broad (SDSS J102912.58+404219.7). wings and an asymmetric peak. The [N ii] lines were also poorly modeled. We then decided to add a medium width component to the [N ii] and Hα emission to match that expected given the Hβ and [O iii] lines. Next, we simultaneously fit the Hβ and Hα brighter than 2MASS in the near-IR follow-up observations by iii ii ΔJ ∼ 1.6, ΔH ∼ 1.45, and ΔK ∼ 1.35 mag. Strong near-IR with three velocity components, and the [O ] and [N ]with excess has been observed in many Type IIn supernovae and has two components. In Figure 5, we present the fits to these lines. been interpreted as thermal emission from dust in a pre-existing In Table 4, we present the flux, velocity, and central wavelength circumstellar nebula (Gerardy et al. 2002). If CSS100217 is a values obtained for the emission lines including both models for Type IIn supernova, near-IR emission is expected to increase at the Hα region. first and then gradually decline over the coming years. In Figure 6, we present the standard Baldwin et al. (1981, The host galaxy, SDSS J102912.58+404219.7, was serendip- hereafter BPT) emission-line diagnostic diagrams used to sepa- itously observed by the GALEX All-Sky Imaging Survey on rate AGNs from starburst galaxies. We include points using the 2004 January 24 with FUV = 19.52 ± 0.17 and NUV = 18.97 ± fit values given in Table 4, along with the emission line galaxies 0.08 mag, and by the Medium Imaging Survey on 2010 January presented in the MPA/JHU version of the SDSS DR4 catalog 29 with NUV = 17.078 ± 0.009 mag (Gezari et al. 2010). A (Kauffmann et al. 2003; Adelman-McCarthy et al. 2006). Here request for follow-up GALEX observations was made after these we have selected galaxies largely following Kewley et al. (2006). archival observations revealed that the event had brightened in That is, only galaxies having emission lines with S/N > 3in the NUV filter by 1.9 mag, ∼1.5 months before the optical each line species and S/N > 20 in the spectrum were selected. Galaxies were selected in the redshift range 0.04

7 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Figure 5. Host galaxy emission-line fits. Dotted line: narrow components of Hβ ,Hα,and[Oiii]. Long-dashed line: intermediate velocity components. Short-dashed line: broad Hα and Hβ components. Solid line: overall fit. (A color version of this figure is available in the online journal.)

(2006), only 32 lie in the starburst region of Kauffmann et al. Table 4 SDSS Spectrum Emission Lines (2003)in[Oiii]/Hβ versus [N ii]/Hα and 16 lie in this region for the stricter starburst–AGN separation line of Stasinska et al. Feature Flux λ (2006). Clearly the host is not a typical NLS1. Indeed, the (1E-17 erg cm−2 s−1)(Å) location of the object to the left of the AGN–starburst composite Hβb 750 4863.0 objects suggests that the amount of star formation is significant. H 300 4861.5 Three similar objects were investigated by Mao et al. (2009). In βm Hβn 297 4864.6 that case one of the three objects was within the starburst region [O iii]4959m 160 4956.8 on the diagrams, whereas two were within the composite region [O iii]4959n 134 4963.2 in the [O iii]/Hβ versus [N ii]/Hα diagram. Mao et al. explain the [O iii]5007m 470 5003.9 reason for the locations of these objects as AGNs buried in H ii [O iii]5007n 400 5010.8 regions where they are transitioning from a starburst-dominated [O i] 81 6306.6 phase to an AGN-dominated phase. [S ii] 75 6722.1 ii The presence of broad Balmer components within the SDSS [S ] ... 6736.2 spectrum is clear and strongly suggests the presence of an Hαb 1400 6564.4 Hαm 1490 6563.7 AGN. Core-collapse Type II supernovae and stellar winds from H 900 6567.5 massive stars can also produce such broad lines. Izotov et al. αn [N ii]m 250 6548.5 (2007) discussed the possibility of multiple supernova events [N ii]n 163 6552.8 and massive star-forming regions and noted that broad lines can arise from these sources. The SDSS spectrum was taken Notes. Subscripts b, m,andn denote the broad, medium, and on 2003 December 29, while our first CSS observations of the narrow components, respectively. The FWHM fit values for these object were in 2004 December. If a supernova had occurred in components are 2899, 911, and 376 km s−1, respectively. The the galaxy in 2003, it would have faded by the time we first velocity measurements are corrected for instrumental dispersion. observed it. However, the likelihood of such an event is not large. Mao et al. (2009) also discussed this possibility for their the low likelihood of observing two Type IIn supernovae in the three objects. same galaxies within a period of a few years suggests that the The NLS1 nature of the host galaxy is further supported by the SDSS source is an NLS1 and also has significant ongoing star [Ne v] λ3426 emission line. NLS1 exhibit strong Fe ii emission formation. features as seen in the SDSS spectrum. However, Type IIn supernovae also exhibit strong Fe ii lines in late spectra. The −1 3.2.2. Follow-up Spectroscopy width of the broad Hα and Hβ emission lines is ∼2900 km s . −1 This is significantly broader than the 2000 km s limit expected Following the discovery of CSS100217 we immediately for NLS1. The observed line width is consistent with that scheduled spectroscopic observations with the IGO 2 m tele- observed in the late time spectra of Type IIn. Nevertheless, the scope. These observations were taken on February 18 and combination of the line diagnostics and spectral features with showed an outburst spectrum similar to the archival SDSS

8 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Figure 6. Comparison of the host galaxy abundances relative to the SDSS emission-line galaxies. The large solid dot shows the location of CSS100217 based on values from Table 4. Left: the large dots give the locations of known NLS1 galaxies from Zhou et al. (2006). The dashed line shows the Kewley et al. (2001) theoretical division between starburst and AGN galaxies. The solid line shows the division found by Kauffmann et al. (2003) and the long-dashed line shows the division determined by Stasinska et al. (2006). Right: the dashed line shows the Kewley et al. (2001) theoretical division between starburst and AGN galaxies. (A color version of this ﬁgure is available in the online journal.)

location near what was suspected to be an AGN. In order to confirm the initial result we scheduled additional IGO spectra and took a Palomar 5 m spectrum on March 15. The Palomar spectrum was found to be very similar to the initial IGO one but exhibited a slightly shallower continuum slope. Subsequently we obtained spectroscopic follow-up with IGO, Apache Point Observatory (APO), MDM, and Keck. These spectra do show signs of a new broad component not present in the SDSS spectrum. In Figure 8, we present follow-up spectra of CSS100217 spanning the period from 2010 February 18 to May 18. In Figure 9, we present these same spectra after subtracting the archival SDSS spectrum of the host galaxy. A test for whether CSS100217 may be due to AGN activity can be derived from changes in the emission-line characteristics. Narrow emission lines are potentially more useful for this test than broad emission lines because they are powered by the average ionizing flux over decades, rather than over days for broad emission lines (Halpern et al. 2003). To examine the spectroscopic changes in more detail, for each spectrum, we subtracted the fits to the SDSS emission lines from the Hα and Hβ line profiles and fit the residual emission lines. In Table 5, we present the fluxes and line widths for the residuals. iii Figure 7. Comparison of the host galaxy abundances relative to the SDSS The removal of the constant [O ] lines from the host galaxy was emission-line galaxies. The large solid dot shows the location of CSS100217 found to be relatively complete for IGO, Palomar, MDM, and based on values from Table 4. The lines show the Kewley et al. (2006) division Keck spectra. This suggests that the flux calibration is good. We between Seyfert, liner, and starburst galaxies. believe that the errors in the remaining emission-line fluxes are (A color version of this figure is available in the online journal.) ∼20% due to calibration uncertainties. However, the measurements may be higher for Balmer components since the CSS and spectrum. The spectrum clearly exhibited a much bluer con- SDSS photometry suggests that the AGN was in a lower state tinuum and Balmer lines that were stronger by a factor of ∼5, when it was spectroscopically observed by SDSS. The spectra consistent with a Type II supernova. Upon subtraction of the with uncertain flux calibration were not analyzed in this work. SDSS spectrum from IGO data there was no evidence for a In Figure 10, we display the fit to the residual Hα flux change in [O ii] and [O iii] and Fe emission lines. However, the from our November 9 Palomar spectrum when the event had spectrum did not appear to exhibit a significant new broad Hα faded significantly. Clearly, unlike the SDSS spectrum, the component relative to the SDSS spectrum as expected for IIn emission exhibits a strong broad component. However, the supernovae. Interpretation of the event was complicated by its narrow component is of the same width as the SDSS spectrum

9 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Table 5 CSS100217 Emission-line Properties

Observation MJD Phase Hβb Hβm Hβn Hαb Hαm Hαn FWHM (km s−1) IGOFeb 55245 −5 2106 ... 465 2814 1126 323 P200Mar 55270 20 2614 ... 610 3190 1126 323 MDM 55320 70 3630 2106 435 ... 1219 320 Keck 55334 84 4357 1423 407 4879 1219 347 P200Nov 55509 164 4304 1772 405 3843 1406 328 Flux (10−17 erg cm−2 s−1) IGOFeb 55245 −5 2105 ... 1195 4136 1126 1259 P200Mar 55270 20 2210 ... 1334 4136 1126 1888 MDM 55320 75 2280 1228 1501 ... 1219 2727 Keck 55334 84 2315 842 438 7031 1219 2937 P200Nov 55509 164 2877 2105 702 8686 1406 1888

Notes. Balmer emission-line velocity widths in km s−1. Phase is taken relative to the maximum light at MJD 55250. Subscripts b, m,andn denote the broad, medium, and narrow components, respectively. Velocity measurements are not corrected for instrumental dispersion. Palomar spectra taken in 2010 March and 2010 November are noted as P200Mar and P200Nov, respectively, and the IGO spectrum from 2010 February is noted as IGOFeb.

Figure 8. Spectra of CSS100217 at τ = 1 (IGO), τ = 26 (P200), τ = 76 (MDM), and τ = 90 days (Keck) after discovery. Maximum light occurred at Figure 9. Host-galaxy-subtracted follow-up spectra of CSS100217, taken τ ∼ 6. Data shown were obtained with the IGO 2 m, Palomar 5 m + DBSP, between 2010 February 18 and May 18, as per Figure 8. MDM 2.4 m, and Keck 10 m + LRIS. (A color version of this ﬁgure is available in the online journal.) (A color version of this ﬁgure is available in the online journal.)

spectra is likely due to the host flux being incompletely and the medium component is significantly narrower than that subtracted. Slight differences between the SDSS and follow- seen in the SDSS spectrum. The flux ratio of Hαb to Hαn in the up spectra may also arise from the 3 diameter fiber used by late Palomar data is 4.6 after subtracting the SDSS contribution. SDSS. This fiber is large enough to contain the bulk of the light This is very different from that found in the SDSS spectrum from the host, whereas long-slit spectra with widths ∼1 were (1.4). The total Hα flux is observed to increase with time, while used for transient follow-up observations. However, our Hubble the continuum and optical luminosity decreases. Both Hβ and Space Telescope (HST) images suggest that almost all of the −1 Hα exhibit a new broad component with width ∼4000 km s flux from the host and CSS100217 lies within the central 1 .No that increases in velocity with time. However, unlike Hα,Hβ second contributing source is seen within 3 . shows little change in total flux with time. In addition to optical spectra we obtained GALEX NUV grism As noted above, the light curve (Figure 1) shows an increased observations on 2010 April 17 and 2010 April 29. We detected brightness at the end of 2004. This variation is interpreted emission near rest wavelength 1910 Å which likely corresponds as largely due to AGN activity and suggests that the source to [C iii](λ1909). This emission is clearly detected and has an − was fainter at the time that the SDSS spectrum was observed. intrinsic width of 3900 ± 500 km s 1. Such emission is seen in Therefore, some of the flux observed in our SDSS-subtracted NLS1 galaxies (Leighly & Moore 2004) but is also associated

10 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Table 6 Correction Parameters

Spectrum Phase BBT KV KR bV bR IGO −5 16.2 ± 1.8 −0.13 −0.11 23.6 25.1 P200 20 13.5 ± 0.8 −0.01 0.04 15.2 14.2 APO 45 12.5 ± 0.8 −0.08 0.12 14.8 12.1 MDM 70 11.5 ± 1.2 0.02 0.08 12.2 10.0 Keck 84 7.8 ± 2.0 0.21 0.36 7.4 4.1

Notes. Column 1 gives the source of the spectrum used to determine corrections. Column 2 gives the time of the spectrum relative to the adopted maximum light at MJD 55250. Column 3 gives the blackbody ﬁt temperature in kK. Columns 4and5givetheK-correction in V and R bands, respectively. Columns 6 and 7 give the bolometric correction in V and R bands, respectively.

At this temperature much of the energy is emitted at rest- frame wavelengths shorter than the observed in optical spectra (λ<3440 Å). At later times the blackbody emission component has cooled to ∼8 × 103 K and far less flux is emitted at blue wavelengths. Therefore, in order to determine the bolometric correction, we must account for this emission and its evolution. We fit the continuum component of the host-galaxy-subtracted spectra taken between Tp − 5 and Tp + 84 with a blackbody. Figure 10. Fit to outburst Hα emission observed with Palomar 5 m on 2010 November 9. The red line shows the SDSS profile and the black line the Palomar The linear fit to the temperature of the blackbody component −1 data after subtracting the SDSS fit. The green line shows the three-component in these spectra gives a cooling rate of 73.0 ± 0.5 K day .At fit. The dashed blue line shows the broad Hα component. the time the second Palomar spectrum was taken, Tp + 164, (A color version of this figure is available in the online journal.) the amount of additional continuum flux from CSS100217 is too small to provide an accurate blackbody fit, so we adopt the with circumstellar emission and ejecta in Type IIn supernovae Keck values. (Fransson et al. 2005; Cooke et al. 2010). We did not detect any To approximate the full SED for the event, we combine variability over the 12 days between the two epochs. our observed spectra with the blackbody for wavelengths λ< 3440 Å. We then integrate the complete model fluxes and those 3.3. Energetics of CSS100217 expected with response expected within the V and R passbands. Since we do not know the exact response of the unfiltered The extreme luminosity of CSS100217 is a clear sign of the CSS data, we cannot determine accurate K-corrections for VCSS energy powering this event. In order to determine the amount photometry. In Table 6, we present the bolometric corrections of optical energy expended we follow the calculations of the and K-corrections determined from the SED models. At early energetic Type IIn supernova SN 2003ma given by Rest et al. times the bolometric correction is large because of the hot (2011). The bolometric luminosity in band X can be defined as component and at later is much closer to values from a − solar SED. = (M,X (MX )/2.5 Lbol,X bXL,X10 , (1) In order to determine the bolometric luminosity of the event we correct the filtered V and R photometry (from ARIES, where the solar constants for filters X = (V,R)areM,X = 32 −1 Table 3) for extinction and K-corrections from the spectra (4.83, 4.42) and L = (4.64, 6.94) × 10 erg s , respec- ,X taken nearest the observing time. In all cases these corrections tively (Binney & Merrifield 1998). The absolute magnitude of are small. To determine the total bolometric luminosity we the event in filter X, M , is determined from the corrected pho- X integrated the values of L and L during the period tometry and b is the bolometric correction. As noted in the bol,V bol,R X when the filtered and unfiltered photometry completely overlap previous section, the event exhibits significant spectroscopic (Tp+40toTp+ 230). The resulting values are Ebol,Vp = 3.6 × evolution during the event. Thus, the bolometric correction and 51 51 10 erg and Ebol,Rp = 2.9 × 10 erg. The subscript p denotes the K-correction applied to determine MX vary with time. In order to account for this variability, we use the IGO, P200, APO, that this is only for the overlapping period. The corresponding = × 50 MDM, and Keck calibrated spectra from Table 1. The spectra values without bolometric correction are EVp 3.9 10 = × 50 are spread out over intervals of roughly 20 days, accounting for and ERp 4.4 10 erg, respectively. The corresponding ¯ 90 days when the event was emitting the most energy near peak average bolometric corrections over this period are bV = 9.3 ¯ luminosity. K-corrections are determined for V and R filters by and bR = 6.6. integrating the event spectra combined with the known filter In order to estimate energy values for the entire event transmissions. As the redshift is relatively small, the spectra we first assumed the V-band corrections for CSS photometry cover both the rest and observed V-passband wavelength range covering the entire VCSS light curve. The total integrated = × 52 = and we calculate in-band (Vobs to Vrest) K-corrections, rather bolometric luminosity is Ebol,VCSS 1.3 10 erg and EVCSS 50 than cross-band corrections (e.g., Robs to Vrest). 8.5 × 10 erg without bolometric correction. The bolometric The early spectra of CSS100217 show the clear presence of correction averaged over the complete CSS light curve is ¯ ∼ a hot continuum component that is a good fit to a blackbody of bVCSS 15. This factor is large due to much of the energy temperature 1.6 × 104 K in the rest frame. The GALEX UV being expended at short wavelengths near peak when the event photometry from 2010 January 29 also supports this result. had a temperature ∼1.6 × 104 K. Nevertheless, as these data

11 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al. lack an accurate K-correction we favor the values determined from the filtered ARIES photometry. Next, we determined the integrated luminosity for the CSS data taken during the period when both filtered and unfiltered observations were made (Tp +40toTp + 230) and found = × 51 Ebol,VCSSp 3.8 10 erg. This measurement is very close to 51 the value from filtered V photometry (Ebol,Vp = 3.6×10 erg). The ratio of energy from the full CSS light curve (Ebol,V )to ∼ CSS Ebol,VCSSp is 3.5. This provides an estimate of the fraction of energy expended during the entire event relative to the period when filtered observations were taken. We thus estimate the total bolometric luminosities for the filtered photometry by multiplying by this factor (∼3.5). We 52 52 find Ebol,V ∼ 1.3 × 10 and Ebol,R ∼ 1.0 × 10 erg. Since we have applied the correction based on the CSS light curve data, the average bolometric correction over the entire event is approximately the same as for the CSS data (∼15). The correction factor is larger than has been found for other supernova of similar temperature. The reason for this likely due to the difference in the SED from other types of supernovae. The uncertainty in these values is expected to be of order 25% due to photometric errors, sparse sampling of the CSS light curve near peak, variation of the spectra with time, uncertainty Figure 11. Light curve of CSS100217. Here, we show the luminosity of transient in spectroscopic flux calibration, and variations in the flux ratio event CSS100217 compared to the luminous and energetic Type IIn SN 2006gy for filtered and unfiltered response. As these measurements are from Smith et al. (2007; open triangles). The CSS100217 light curves are given for VCSS, solid line with open boxes; I short-dashed line; R dot-dashed line; based on 287 rest-frame days it is likely that a slightly higher V dotted-line; and B long-dashed line. The maximum brightness occurred at value would be found for the full light curve. MJD = 55250 which corresponds to ∼6 days after discovery. See the text for In comparison to CSS100217, a number of very energetic further details. Type IIn supernovae have recently been discovered. Rest et al. (A color version of this figure is available in the online journal.) (2011) found the past Type IIn supernovae, SN 2003ma, expended 4×1051 erg (with a bolometric correction of ∼3.4) and × 51 Drake et al. (2010a) obtained a value of >1.4 10 erg (without Time with the HST WFC3. We obtained WFC3 images in bolometric correction) for the Type IIn supernova SN 2008fz. F390W, F555W, and F763M filters spanning one orbit. The Similarly, Kozlowski et al. (2010) found that the Type IIn 51 filters were chosen to separate the blue continuum and Hα supernova, SDWFS-MT-1, expended >10 erginthe4.5 μm components of the spectra, since we expected a Type IIn Spitzer/IRAC band. Smith et al. (2010a) found that the Type IIn supernova would be a strong source of Hα and this might enable a supernova SN 2006gy had an integrated bolometric luminosity clearer separation from the hot blue central AGN. A customized × 51 of at least 5 10 erg. In Figure 11, we plot the filtered seven-pointing dither was chosen in each band to maximize the and unfiltered absolute magnitude light curves of CSS100217 subsampling of the HST PSF (Anderson & King 2000). along with that of the Type IIn supernova 2006gy (Smith et al. In Figure 12, we show a co-added SDSS image of the host 2007). Considering the difference absolute luminosity between galaxy along with the dithered HST WFC3 F555W image of these two events (δR > 1 mag), the approximate factor of two the host including transient CSS100217. The SDSS pre-event difference in energy appears as expected. co-add consists of the median of 15 images (u,g,r,i,z)ofthe 3.4. High-resolution Imaging galaxy observed on three nights (2002 December 31, twice, and 2003 March 26), whereas the HST image is derived from In order to discern whether CSS100217 was due to an event seven dithered images. The SDSS and HST observations show occurring in the nucleus of SDSS J102912.58+404219.7, as that the galaxy is completely dominated by the central nucleus required for a TDE or other AGN variability, we investigated with no clear sign of extension of the host galaxy. Subtraction the location of the event. First, we determined the location of of HST PSF models did not reveal the galaxy or any sign of the flux in the difference images relative to the centroid of a second separated source. In comparison, Deo et al. (2006) SDSS J102912.58+404219.7. Such astrometry can be used to examined HST data for a sample of 87 Seyfert galaxies and disambiguate variable sources that are not resolved because found almost all occurred in clear spiral galaxies. Only two of blending or high background light (Nelson et al. 2009). objects appeared point-like (HEAO 2106-099 and Mrk 335). A significant offset between the two centroids would very However, Bentz et al. (2009) examined HST data for 35 AGNs, strongly suggest that the object was a supernova. By stacking including Mrk 335, and were able to fit the faint host galaxies the sequence of difference images we found that the additional for all. It is therefore likely that the host galaxy simply has a flux was within 0.3 of the galaxy’s nucleus. In order to very low surface brightness relative to the nucleus. check this result we undertook follow-up observations with the We obtained images of the object using the NIRC2 imager Large Binocular Telescope plus LUCIFER near-IR camera and and laser guide-star adaptive optics (Wizinowich et al. 2006)at spectrograph. No second source was resolved in our Ks-band the Keck II 10 m telescope on Mauna Kea, Hawaii, on 2010 images with resolution of 0.4. June 3 UT. We used the K band and obtained multiple dithered To obtain a higher level of precision, and possibly separate the exposures both in the NIRC2 “wide field” mode, with a sampling source for the AGN, we obtained HST Director’s Discretionary of 40 mas pixel−1, and the “narrow field” mode, with a sampling

12 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Figure 12. SDSS image of the host galaxy of CSS100217, SDSS J102912.58+404219.7 (left) and HST F555W image of the event during outburst (right). The images are 15. 5 across with north up and east to the left.

Figure 13. Keck NIRC2 images of CSS100217 with north up and east to the left. Left: the “wide field” image with scale ∼15 across. Right: the “narrow field” image with scale ∼10 across. of 10 mas pixel−1, followed by observations of a nearby bright Table 7 star to define the PSF. For our tip-tilt configurations and seeing EVLA Radio Measurements conditions, the estimated K -band Strehl ratio is ∼0.2. A co-add 2010 UT Date Flux 4.5 GHz Flux 7.9 GHz α of a number of dithered images is shown in Figure 13.Atthe (μJy) (μJy) redshift of the transient, the pixel sizes correspond to projected ± ± − ± ∼ ∼ Apr 29.22 447 26 399 24 0.20 0.02 linear sizes of 103 and 26 pc. Thus, the effective angular May 14.07 312 ± 24 349 ± 24 +0.19 ± 0.02 resolution of these observations is comparable to or slightly Jun 01.05 408 ± 16 506 ± 28 +0.38 ± 0.03 better than that of the HST images, and the source appears unresolved even with the higher resolution images. and 7.5 at 4496 MHz and 7916 MHz, respectively. Phase 3.5. Radio Observations calibration was carried out by making short observations 3.5.1. EVLA of the nearby point source J1033+4166 every 10 minutes, while amplitude and bandpass calibration was achieved using an We observed CSS100217 with the Expanded Very Large observation of 3C 147 at the end of each observing run. The data Array (EVLA) for three separate epochs between 2010 April were reduced following standard practice in the Astronomical and May. Each observation was 1 hr in duration. We ob- Image Processing System (AIPS) software package. served simultaneously at central frequencies of 4496 MHz A single, unresolved radio source was detected on all three and 7916 MHz using the new wide C-band feeds for a to- epochs. Table 7 summarizes the results of these observations. tal bandwidth of 256 MHz. The EVLA was in the com- The flux density varied between 0.3 and 0.5 mJy, but the spec- pact D configuration, yielding synthesized beams of 13 trum was relatively flat between these two radio frequencies.

13 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al. Inspection of the archival FIRST images made with data taken CSS100217 using data from the Large Area Telescope (LAT) in the early 1990s (Becker et al. 1995) shows a possible detection aboard the Fermi Gamma-ray Space Telescope satellite. The at this position with a 1.4 GHz flux density of 300 ± 145 μJy. Fermi-LAT instrument is sensitive to gamma-ray photons with At the redshift of z = 0.15 (dL = 705 Mpc), these flux densities energies in the range 20 MeV to >300 GeV. With its ∼2.4sr 29 −1 correspond to a spectral luminosity LR 2.3 × 10 erg s field of view (FoV) and scanning mode of operation, Fermi- Hz−1. The variation in spectral index of the 4.5 GHz and 7.9 GHz LAT provides all-sky monitoring coverage on 3 hr timescales data suggests that the distribution is getting flatter and eventually (Atwood et al. 2009). inverting. The origin of this effect is unknown. We extracted data within a 20◦ acceptance cone centered on The radio luminosity of CSS100217 plus the host galaxy the CSS100217 position during the period 2009 November 20 to exceeds that of the most luminous Type Ib/c and II supernovae 2010 July 4, covering the nominal duration of the outburst. We by factors of several (Chevalier et al. 2006) and is approaching analyzed these data using the standard Fermi-LAT data analysis values more typical of GRB afterglows. The absolute near- software, running the unbinned maximum likelihood analysis IR luminosity from 2MASS is MK =−24.65. Mauch & tasks; we fit the data over 1 week intervals and over the entire ex- Sadler (2007) studied the properties of star-forming galaxies traction period. (See Abdo et al. 2010a for descriptions of the fit- and radio-loud AGNs. The combination of the 2MASS K-band ting methods.) The source model for these analyses comprised a luminosity and initial FIRST radio power places the object in power-law point source at the CSS100217 position (with uncon- the region of overlap between both star-forming galaxies and strained photon index and normalization), all point sources from AGNs. Based on the local luminosity function star-forming the 1FGL catalog (Abdo et al. 2010a) within the data extraction galaxies are slightly more common at this radio power (Mauch & region, and the models for the Galactic (gll_iem_v02.fit) Sadler 2007). However, during outburst the near-IR luminosity and isotropic diffuse emission (isotropic_iem_v02.fit) that is Mk =−26, making it brighter than most star-forming radio were recommended by the Fermi-LAT team. The software and galaxies. Additionally, when considered with the flat spectral diffuse files are available from the Fermi Science Support Cen- index and the modest variability, the simplest explanation is ter (http://fermi.gsfc.nasa.gov/ssc/). The flux normalizations of that the radio emission originates from nuclear activity from the the Galactic and isotropic components, the point sources within central black hole in the galaxy. 5◦ of CSS100217, and the flaring blazar Mrk 421, which is 7◦.25 from CSS100217, were allowed to be fit freely. All other 3.5.2. GMRT point-source parameters were held fixed at their 1FGL catalog values. No evidence for a point source at the location of The source CSS100217 was observed with the Giant Me- CSS100217 was found in the full data set or in any of the terwave Radio Telescope (GMRT) on 2010 May 23 at a cen- 1 week intervals. The 95% confidence limit (CL) flux upper ter frequency of 608 MHz. Data were recorded using a new limit in the energy band 100 MeV to 100 GeV for the full software correlator with a bandwidth of 33 MHz divided into data set is 3.1 × 10−8 photons cm−2 s−1. For the 1 week in- 512 channels. The total observing time was 5 hr (including the tervals, the 95% CL upper limits ranged from 1.1 × 10−7 to calibration overheads). A total of 2 hr 45 minutes was spent 3.4 × 10−7 photons cm−2 s−1. Near the time of the peak op- on the target, interspersed with 4.5 minute scans on the phase tical flux (MJD 55272), the 1 week 95% CL upper limit for calibrator every 45 minutes. The source 0834+555, which is an CSS100217 was 2.1 × 10−7 photons cm−2 s−1. We note that unresolved point source at 610 MHz, was chosen as the phase the two nearest point sources, 1FGL J1033.2+4116 (0◦.95 offset calibrator, while 3C 147 and 3C 286 were used to calibrate the from CSS100217) and 1FGL J1023.6+3937 (1◦.52 offset), are flux as well as the bandpass. Their fluxes were determined us- both associated with blazars (Abdo et al. 2010b). Neither source ing the Baars et al. (1977) flux scale. Data were reduced using showed significant emission during any of the weekly time inter- standard AIPS processing. vals that we considered, and the fitted fluxes for each were <6× The image was made using the central 30.5 MHz centered at 10−8 photons cm−2 s−1 at the time of the peak optical flux of 607.95 MHz. The synthesized beam achieved was 5.78 × 4.21. ± CSS100217. An unresolved point source of flux 873 80 μJy was detected. Since luminous optical transients such as Type Ib/c super- The rms near to the source was about 50 μJy, which implies a novae have been associated with GRBs (Paczynski 1997; Stanek detection significance of about 17σ. The combination of EVLA et al. 2003), we considered the possibility that CSS100217 was data with GMRT suggests a spectral slope α between −0.4 and − associated with such an event. We queried the GCN Notices 0.5 depending on EVLA observation date. This is similar to archive (http://gcn.gsfc.nasa.gov/) for any bursts that occurred values commonly observed in low-luminosity Seyfert galaxies around the time of the onset of the optical flare. From 2009 (Ulvestad & Ho 2001). November 20 to December 29, GCN Notices for 49 GRB triggers were issued. Of those 49 triggers, the closest GCN lo- 3.6. X-ray Observations cation to CSS100217 was 14◦.6 away (R.A., decl. = 155◦.667, ◦ X-ray observations of the transient were taken on 2010 April 55.233, J2000) for trigger number 281250934 issued by the 6 with the Swift X-Ray Telescope (0.2–10 keV) to determine Gamma-ray Burst Monitor (GBM) aboard Fermi with trigger time 05:15:32.98 UT, 2009 November 30. The other 48 GRB whether the object was a strong soft X-ray source as expected for ◦ a TDE. The object was clearly detected with flux ∼1043 erg s−1 candidate locations were more than 20 away from CSS100217. and is a soft source consistent with a TDE or an NLS1 galaxy No obvious transient at the CSS10027 location is present in the (Boller et al. 1996). LAT data stream in the 200 s bracketing the GBM trigger time. However, at the time of the trigger, the GBM location was at ◦ 3.7. Fermi-LAT Follow-up the edge of the LAT FoV (off-axis angle of 70 ). So to search for possible extended, longer timescale emission (e.g., Abdo We searched for emission in the gamma-ray band that was et al. 2009), we extracted data centered on the CSS100217 po- spatially and temporally coincident with the optical flare from sition for the 104 s following the GBM trigger. An unbinned

14 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al. likelihood analysis of those data finds no evidence for a point the blast shock travels. The location of the event near the AGN source and yields a 95% CL flux upper limit of 3.7 × 10−6 may be responsible for causing additional heating of the CSM photons cm−2 s−1 for energies 100 MeV to 100 GeV. surrounding a massive progenitor star. 4.1.1. Supernovae in Nuclear Regions 4. THE NATURE OF THE TRANSIENT CSS100217 Modern optical searches for supernovae have been tuned to Determination of the nature of the transient CSS100217 avoid events occurring near galactic nuclei. The main reason for bares distinct similarities to that of the transient SDSS this is the high likelihood that such variability is due to variation J095209.56+214313.3 discovered by Komossa et al. (2008). caused by an AGN. AGNs are thought to be present in 43% Following the initial interpretation of SDSS J09522.56+2143 as of galaxies (Ho et al. 1997). Therefore, the chance of finding a a TDE by Komossa et al. (2009) obtained additional data and variation in any given galaxy due to an AGN may be larger than reinterpreted their discovery as either a TDE, AGN variability, the chance that this is due to a nuclear supernova. In addition, or a luminous Type IIn supernova. In this case we have the same supernovae lying far from the crowded cores of galaxies can also three likely causes for the observed outburst. However, unlike be more readily discovered and spectroscopically confirmed. SDSS J095209.56+214313.3, which was only discovered and Furthermore, supernovae in the dense cores of regular galaxies followed 2 years after the outburst, we obtain a spectrum before are likely to suffer from significant extinction (AV > 10 in many the event and were aware of the distinctive nature of the event cases), so that few are visible in optical wavelengths. However, from well before it reached its optical peak. We were thus able in the cores of luminous infrared galaxies, such as the interacting to obtain significant data covering the event. Nevertheless, the system Arp299, at least one CCSN is expected every year interpretation of CSS100217 remains unclear. Below we will (Mattila et al. 2004). Such supernovae can be discovered using outline the cases for and against each of the possibilities. near-IR imaging or radio observations (Perez-Torres et al. 2007). Perez-Torres et al. (2007) discovered 26 sources that they 4.1. Type IIn Supernovae believed to be radio supernovae or supernova remnants within As noted above CSS100217 bares distinct similarities to Type the central 150 pc of Arp299A. Additional follow-up by Perez- IIn supernova explosions. Therefore it is useful to compare Torres et al. (2010) showed that one of the central sources was in the event to known sources of this type. Type IIn supernovae fact a low-luminosity AGN. However, Perez-Torres et al. (2010) are known to have a 5 mag range in their peak brightness identified one companion source within a projected separation and outburst timescales from months to years (Richardson of 2 pc as a radio supernova. In many cases, the supernova et al. 2002). In recent years luminous and energetic Type IIn observed near the core of a galaxy will lie in front of the supernova, such as SN 2006gy (Quimby et al. 2006), have dust extinction layer making them visible in optical surveys. been discovered and analyzed (Smith et al. 2007). Comparison Additionally, supernova lying within the narrow-line region near between the light curve of CSS100217, and the energetic an AGN (10 pc to 1 kpc) may not be obscured by the dense gas and dust disk. Botticella et al. (2008) identified 28 supernova Type IIn supernova 2006gy (Smith et al. 2007)isgivenin Figure 11. The similarity of these curves suggests a possible candidates lying with 0.5 of the core of their host galaxies. Of relation between the two events. Additional Type IIn supernova these only 50% were attributed to AGNs based on their long with very long timescales have also been recently discovered. timescale variability. For example, SN 2008iy (Catelan et al. 2009) and 2003ma Strubbe & Quataert (2010) studied how one could discern (Rest et al. 2011) have been observed over many years. The nuclear supernovae from TDEs. They found that HST or ground- decline rate measured from our difference image photometry based adaptive optics were needed to reduce contamination of for CSS100217 at late times is 0.0175 mag day−1. This is much supernovae near the galactic nucleus to a level close to that faster than observed for the longest events, but slower than expected for TDEs. However, even with high-resolution imaging observed for most Type IIn supernovae. the number of Type Ia and II supernovae will exceed the number Additionally, the spectra of CSS100217 show evolution of of TDEs. the broad Balmer components. In the initial IGO spectrum the −1 4.2. Tidal Disruption Events Hα component had FWHM ∼ 2800 km s and 3 months later during the Keck observations the width was ∼4800 km s−1.This The search for TDEs has recently led to the discovery of a is close to that observed for Hα in SN 2006gy which initially number of candidate TDEs (Komossa et al. 2002; Esquej et al. had FWHM = 2500 km s−1 and later 4000 km s−1 (Smith et al. 2007; Gezari et al. 2009a; Cappelluti et al. 2009; Maksym et al. 2007). The Hβ emission shows the similar evolution. In the case 2010). Dynamical models of galaxy nuclei predict that these of CSS100217, as in known Type IIn supernovae, we see that events should occur at a rate of 10−4 to 10−5 galaxy−1 yr−1 the Hα emission component strengthens with time, while the (Magorrian & Tremaine 1999; Wang & Merritt 2004). In observed luminosity declines. This suggests that the emission comparison these events are 100 times less commonly observed will continue for an extended period of time. The spectrum of than supernovae, but still within the range of transient surveys CSS100217 exhibits relatively strong high-energy Balmer lines such as CRTS, PTF, PanSTARRS, and LSST. (Hγ ,Hδ,H ) that are more commonly observed in luminous The signature of such events is bright UV and soft X-ray blue variable (LBV) outbursts like SN 2009ip (Smith et al. emission from the high-temperature outburst event that occurs as 2010b). This may be attributed to the surrounding medium being the star is shredded and accreted by a massive black hole. Models hotter than normally observed for Type IIn supernovae. The for the optical light curves of these events show significant emission features observed in Type IIn supernovae are due to the variation with black hole mass. TDEs are expected to have expansion of explosion ejecta into a dense circumstellar medium temperatures of ∼105 K and a luminosity that declines with time (CSM) surrounding massive eta Carina-like stars (Smith et al. as t−5/3. However, recent detailed models for the light curves of 2010a). In such cases, the outbursts of LBVs in years prior to the these events by Strubbe & Quataert (2009) and Lodato & Rossi explosion are believed to cause massive shells through which (2011) found the optical and UV light curve to be significantly

15 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al. shallower than t−5/3. Lodato & Rossi (2011) find a very slow AGNs, we decided to investigate the light curves of a large, uni- t−5/12 decline at late times. form sample of spectroscopically confirmed NLS1 galaxies. We Although the clearest signature for TDEs is a flare near first selected the spectroscopically confirmed sample of NLS1 the center of an otherwise dormant galaxy, such events are from Zhou et al. (2006). This set consists of ∼2000 NLS1, cho- very likely to occur in galaxies that exhibit significant nuclear sen from emission-line galaxies in the SDSS dr5 catalog. We activity from black holes in the right mass range. In particular, found matches to most of the Zhou et al. NLS1s in the CSS data supermassive black holes seen in blazars and QSOs are thought archive and for each object we extracted the photometry from to be too massive for such disruption events to occur (Hills the same 5 year period as CSS100217. Many of the light curves 1975). Instead, in these cases the stars are swallowed whole. show clear, significant variation over this period. 8 For black holes with masses <10 M, as in Seyfert galaxies, After removing the NLS1 light curves that were affected by such events should occur. Clearly the presence of X-ray, UV, blends or bad photometry (due to being on image edges, etc.), we and radio variability in AGNs make the discovery of TDEs were left with 1541 objects. We matched the objects against the FIRST radio catalog using a 5 radius. Of the 143 matches, only associated with AGNs a much more complex task. The slow rise and decline of the optical light curve of 3 had an offset larger than 2 . The NLS1 host of CSS100217 CSS100217 is inconsistent with the t−5/3 decline expected for was not detected by FIRST or NVSS radio surveys. TDEs (Komossa et al. 1999) as well as the more recently For each light curve we iteratively determined the mean and theorized t−5/12 dependence. A continuum fit to the IGO range of values for the 90% of data lying nearest the mean. This spectrum shifted to rest gives a temperature of (1.6 ± 0.2) × process removes photometric outliers due to bad photometry 104 K, much lower than the expected TDE value of ∼105 K. caused by artifacts, satellite trails, bad seeing, etc. We then As noted earlier, the NUV photometry taken on 2010 January calculate the standard deviation assuming that the remaining 29 by GALEX gives NUV = 17.08. This value is 1.9 mag data follows the normal distribution. In Figure 14,weplot brighter than when observed on 2004 January 24 (Gezari et al. the scatter of the data points for the NLS1 sample. As the 2010). At this time the object was ∼1.5 mag brighter in CSS scatter is dominated by increasing photometric uncertainty with observations. The combination of extinction-corrected GALEX decreasing brightness we also plot the data after removing this NUV flux (centered at 2315 Å) along with the IGO data is trend. Clearly CSS100217 is an extreme outlier, whether we consistent with a temperature of ∼104 K. However, this early correct for the photometric uncertainty trend or not. temperature is poorly constrained because of the 3 week period To further investigate the variability we determined the between these observations. median magnitude for each light curve and the value that varies Recently, Strubbe & Quataert (2010) studied the spectro- most from the median in terms of photometric uncertainties. scopic signatures of TDE events. The results suggest that most Here, we included only those data points lying within ±3σ as events will give rise to featureless blackbody spectra at wave- determined from the scatter in each light curve. This sigma cut lengths above 2000 Å. In an otherwise inactive galaxy the sig- was necessary because of outliers in other NLS1 photometry, nature of a disruption event by a quiescent black hole would due to artifacts, etc. In Figure 15, we plot the peak variation be similar to the early spectrum of a supernova. However, the versus the number of sigma that the point represents. The strong spectroscopic features would not evolve as time passed. size of the deviations are mainly less than 5σ , suggesting In the presence of an AGN the combination of a featureless spec- that the photometric uncertainties are approximately correct. trum with that of the AGN would be very difficult to discern We also show the result when we have removed the effect of from that of just an AGN. Strubbe & Quataert (2010) note that increasing photometric scatter with increasing magnitude. This TDEs, unlike Type IIn supernovae, become hotter with time and figure shows that CSS100217 is once again an extreme outlier are not expected to produce strong optical emission lines. The in both plots. As the host of CSS100217 is brighter than most of theoretical predictions for spectra, temperature, and variability the NLS1 sample, the offset is most clearly seen in the corrected all appear inconsistent with the observations of CSS100217. plots. We note that the largest deviation (ΔV ) from the median is not shown in the figures for CSS100217 as the true maximum 4.3. AGN Variability lies >3σ from the median magnitude. The level of observed NLS1 variation is consistent with that found by Ai et al. (2010). As the host galaxy appears to contain an AGN, it is important Among the Zhou et al. (2006) spectroscopically selected to consider whether CSS100217 might be due to an accretion NLS1 two are known to exhibit significant variability on event involving the massive black hole. In the previous section short timescales. However, both of these NLS1, SDSS we determined that the event was very unlikely to be due to the J150506.47+032630.8 (QSO B1502+036, Yuan et al. 2008) and tidal disruption of a star by the black hole. However, as AGNs SDSS J094857.3+002225.5 (Zhou et al. 2003), are radio-loud are by nature variable it is necessary to understand the limits NLS1 (380.49 mJy and 111.46 mJy, respectively in FIRST data). of this variability in comparison to CSS100217. Recently, Ai In Figure 16, we present the light curves of these two NLS1. et al. (2010) investigated the variability of a small sample of 58 Both objects exhibit erratic variability which is completely un- narrow-line Seyfert-1s (NLS1s) and 217 broad-line Seyfert-1s like CSS100217 or the other radio-quiet NLS1 examined. In (BLS1s). They found that the variability in 3 years of SDSS these cases, like BL Lacs, the variability has been attributed stripe-82 data was less than 0.4 mag. As the host galaxy for to a jet lying along our line of sight and emanating from the CSS100217 is an NLS1, the sample of Ai et al. (2010)is central black hole. The optical variability is attributed largely too small for a meaningful understanding of the true limits of to this source. The maximum observed variability for these ob- variability in NLS1s. jects in CSS data of SDSS J150506.47+032630.8 and SDSS ∼ ∼ 4.3.1. Comparison of CSS100217 with Known NLS1 J094857.3+002225 is 1.5 and 1.2 mag, respectively. The CSS light curves of these NLS1 resemble those of blazars mon- In order to assess how the variability we observed in itored by CRTS. SDSS J094857.3+002225 exhibits intra-day CSS100217 matches with that naturally observed for NLS1-type variability (Liu et al. 2010). These two sources are ∼500 times

16 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Figure 14. Comparison of the variability of CSS100217 (large dot) with known NLS1s selected from Zhou et al. (2006). Radio sources detected by FIRST are marked with medium-size dots. The location of radio-loud NLS1 SDSS J150506.47+032630.8 and SDSS J094857.3+002225.5 is marked with boxes. Left: standard deviation of the light curve based on central 90% of the data. The dotted line shows the trend of increasing scatter that is mainly due to decreasing brightness. Right: the scatter of the data after removing the trend of increasing scatter with decreasing magnitude. (A color version of this ﬁgure is available in the online journal.)

Figure 15. Variability of CSS100217 (large dot) relative to known NLS1s selected from Zhou et al. (2006). Radio sources detected by FIRST are marked with medium-size dots. The location of radio-loud NLS1 SDSS J150506.47+032630.8 and SDSS J094857.3+002225.5 is marked with boxes. Left: maximum deviation from median magnitude in terms of sigma based on NLS1 data. Right: maximum deviation from median magnitude in sigma based on NLS1 data that has been corrected for increasing scatter with decreasing brightness. (A color version of this figure is available in the online journal.) brighter than CSS100217 at radio wavelengths and neither ex- supernova. This strongly suggests that CSS100217 is not due hibits a prolonged outburst like that observed for CSS100217. to regular AGN variability. However, it is not possible to From our analysis we find that a small fraction of the NLS1 completely exclude very rare sources of AGN variability. Recent exhibit significant variability. In most cases the variability is reports of AGN outbursts have been made in sparsely sampled a slow change in brightness over a timescale of years, as data by Kankare et al. (2010) and Valenti et al. (2010). However, previously observed for QSOs. A couple of NLS1 in our large these have been found to be due to gradual changes rather than sample exhibit a high level of variability but none like that outbursts when examined in better sampled data (Drake et al. observed in CSS100217. In the cases where a rapid event 2010b, 2010c). TDEs themselves are examples of rare AGN lasting months is seen, the source is consistent with a luminous outbursts. Although the characteristics of CSS100217 do not

17 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al.

Figure 16. CSS V-band light curves of two highly variable radio-loud NLS1 galaxies. Left: the light curve of SDSS J150506.47+032630.8. Right: the light curve of SDSS J094857.3+002225.5. follow current theoretical predictions for TDEs, recent revisions consistent with this being an NLS1 galaxy. The HST follow-up of both the timescale (Lodato & Rossi 2011) and spectroscopic of the event shows that the location of CSS100217 is consistent signature of such events (Strubbe & Quataert 2010) suggest a with the location of the bright nucleus. The object is also found need for constraints based on empirical data. Future surveys to be an X-ray source in Swift data, a radio source in EVLA such as LSST should provide these constraints. and GMRT follow-up observations, and is brighter than known supernovae at these wavelengths. 5. DISCUSSION AND CONCLUSIONS It is well known that AGN variability can amount to variations of a magnitude or more over the period of a number of years. We have analyzed the multi-wavelength data covering the As AGNs are more common than supernovae, surveys for discovery of the unusual transient CSS100217. The coincidence supernovae specifically avoid follow-up of detections occurring of the event’s location with an NLS1 galaxy makes the event near the cores of galaxies. Indeed, the IAU recommend that all most consistent with a TDE, AGN variability, or a supernova. supernova candidates are checked against the Veron-Cetty´ and 5.1. CSS100217 as a TDE Veron´ (2010) AGN catalog before submission. Events near the core of galaxies are not announced or given an official ID by the The large outburst of CSS100217 within 150 pc of the nucleus IAU’s Central Bureau for Astronomical Telegrams unless they of an AGN make it a good candidate for the tidal disruption of a are also spectroscopically confirmed to reduce the possibility massive star by the central black hole. The object is also detected that any given discovery is due to AGN variability. The peak as an X-ray source in Swift telescope follow-up. However, the outburst luminosity of CSS100217 is well within the range event exhibits a slow rise over a period of months and a similarly observed for NLS1. One model for AGN variability suggests slow decline. The light curve is completely inconsistent with that the cause is the superposition of supernova explosions −5/3 theoretical TDE models that predict an immediate rise and t in giant stellar clusters (Trelevich et al. 1992). In this model decline. However, the more recent light curve models for TDEs the explosions interact with the high-density circumnuclear (Lodato & Rossi 2011) show optical light curves with rise times environment. If this model is correct, CSS100217 could be an of a month followed by a flat peak and much slower decline example of such an event. −5/12 following t . Such light curves are very similar to those Analysis of host galaxy SDSS J102912.58+404219.7 on the observed for supernovae, although with much longer tails. The BPT diagram suggests that it is not a typical NLS1. The object ∼− peak brightness MV CSS 23 of the event is far greater than lies on the locus of starburst galaxies suggesting that the galaxy theorized for TDEs, which are predicted to reach the brightness is also undergoing rapid star formation. Comparison of the ∼− of regular supernovae MV 18. Furthermore, the galaxy- optical variability of CSS100217 with 1500 other NLS1 galaxies subtracted spectra of the event exhibit strong Balmer emission selected from SDSS data strongly suggests that the observed 4 and a continuum consistent with T = 1.5 × 10 K rather than variability is inconsistent with normal AGN variability. The 5 the theorized 10 K value. Based on theoretical predictions follow-up spectra of CSS100217 exhibits a Balmer component CSS100217 is a poor TDE candidate. that is significantly broader than the archival spectrum. This ∼ −1 5.2. CSS100217 as AGN Variability component is twice as broad as the 2000 km s defining limit for NLS1 galaxies (Osterbrock & Pogge 1985). Furthermore, The host galaxy spectrum of SDSS J102912.58+404219.7 although the narrow and medium Hα velocity components vary exhibits both broad and narrow Balmer lines as well as Fe ii, little in width with time, the strength increased as the event

18 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al. faded rather than decreasing. Variation of the narrow component on short timescales is not expected because of the size of the narrow-line region. The event also shows the presence of a hot continuum component not observed in the prior SDSS spectrum. This component cools as the outburst fades. Furthermore, although there are few examples of supernova occurring near the cores of AGNs, this is due to active selection against such events because of possible AGN variability. Similar events may have been detected in the past but dismissed because of their location near the core of a galaxy and the presence of an NLS1- like spectrum. Based on the observations of NLS1, CSS100217 is unlikely to be due to AGN variability.

5.3. CSS100217 as a Supernova The distinctive smooth rise and fall of the light curve of CSS100217 closely matches the general shapes observed for supernovae and Type IIn, such as SN 2008iy and SN 2006gy. Spectroscopic follow-up of CSS100217 reveals strong narrow Balmer features superimposed on broader features as is required for classification of Type IIn supernovae. The Balmer features are observed to vary significantly from that observed in SDSS spectrum. Some variation in the broad-line strength is expected Figure 17. Comparison between the host-subtracted Keck spectrum of for NLS1 galaxies but such rapid variation in the narrow- CSS100217 and that of Type IIn supernova SN 2008iy. Solid line shows the line strength has not been observed. Additionally the follow- spectrum of CSS100217. Long-dashed line shows the spectrum of SN 2008iy. up spectra exhibit a broader, 4000 km s−1 component that is In both cases the events are observed well after maximum light. not seen in the SDSS spectra. The velocity of this component (A color version of this figure is available in the online journal.) increases with time in both Hα and Hβ until the most recent observations while the narrow and medium components shows are brighter than expected for a supernova, but are consistent little change. Also, the strength of the broad component is seen to with the presence of the NLS1. These detections are below the increase even as the event fades. The host-subtracted spectrum threshold of past radio surveys like FIRST and are therefore ii exhibits strong Fe lines. Such lines are pronounced in both consistent with no change occurring due to CSS100217. The NLS1 spectra and supernova Type IIn like SN 2008iy, SN 2007rt lack of detection in Fermi data does not place a strong constraint (Trundle et al. 2009) and SN 1997ab (Hagen et al. 1997). Such on the nature of this event as the follow-up spectra are not lines are not predicted by current models of TDEs. consistent with broad-line Type Ib/c supernovae that have been As is characteristic of Type IIn (Trundle et al. 2009)the linked to GRBs. Type IIn supernova such as 2006gy have been spectra of CSS100217 do not exhibit the broad P-Cygni features detected in X-rays (Smith et al. 2007) but once again the commonly observed in other Type II supernova. For example, detection of CSS100217 by Swift is consistent with emission the Type IIn prototype, SN 1998Z was determined not to exhibit from the NLS1. a P-Cygni absorption component (Turatto et al. 1993). Narrow Massive η Carinae-like LBVs undergo large outbursts and P-Cygni features are commonly observed in high-resolution data deposit material into the interstellar medium that will eventually of Type IIn supernova, but are not seen in low-resolution data be illuminated when the stars explode. In these cases much of (Trundle et al. 2009). In Figure 17, we contrast the spectrum of the kinetic energy of these explosions is converted to luminosity. CSS100217 with that of SN 2008iy. The brightness limit of such events is mainly constrained by the The temperature derived from the continuum in the initial amount and distribution of circumstellar material interacting = × 4 follow-up spectrum (T 1.5 10 K) is consistent with with the supernova shock. A series of dense circumstellar supernovae in general and very similar to that observed for shells or a surrounding dense medium can give rise to extreme the luminous Type IIn SN 2006gy (Smith et al. 2007). The =− luminosity so that the explosion appears like a supermassive brightness of this event, MV 22.7, is the greater as observed star. Although the optical energy expended by the event is higher for past IIn supernovae SN 2008fz (Drake et al. 2010a) and SN than any past Type IIn supernova, it is within a factor of three 2008es (Gezari et al. 2009b). As Type IIn supernova are known of the bolometric values for SN 2006gy (5 × 1051 erg; Smith to exhibit a variation in peak brightness of at least 5 mag, this et al. 2010a) and SN 2003ma (4 × 1051 erg; Rest et al. 2011). discovery is not very surprising, particularly since the most The presence of strong continuum evolution and ongoing strong luminous among these supernovae have only been discovered Balmer emission, along with a bright and distinctly supernova- in the past decade as surveys have begun which search for like light curve, suggests that CSS100217 is most likely an transients in intrinsically faint galaxies (Drake et al. 2009). extremely luminous Type IIn supernova near the nucleus of an The presence of a luminous supernova near the core of an AGN/starburst galaxy. Further photometric and spectroscopic AGN is expected to be a rare occurrence. However, the host monitoring of CSS100217 should secure the nature of this event spectrum suggests a significant star formation rate, which would and also give further insight into the nature of the host galaxy. enhance the rate of Type II supernova—as seen in Arp299A by Perez-Torres et al. (2007). Additionally, based on Strubbe & 5.4. A Supernova Associated with the AGN? Quataert (2010), the rate of Type II nuclear supernovae will exceed the rate of TDEs by a factor of ∼3at0.05 resolution for A key piece of evidence is the observed proximity of the 7 a10 M black hole. The radio detections by GMRT and EVLA transient to the AGN of the host, since both the HST and

19 The Astrophysical Journal, 735:106 (21pp), 2011 July 10 Drake et al. the Keck AO observations indicate a single, unresolved point regions of intrinsically luminous galaxies (Drake et al. 2009). ∼− source, consistent with a single PSF. It is worth noting that For this reason only events brighter than MVCSS 21 would they span a range of ∼5–6 in wavelength, so that the absence have been detected in this particular host galaxy. However, an of a second point source in either one cannot be attributed event as luminous as CSS100217 would be detected at any to a hypothetical extreme difference in colors. For the HST, location within almost any galaxy to the distance of CSS100217. Sparrow’s resolution limit is 0.043. The resolution of the Keck We will report on a systematic search for more such events in AO images is comparable, with 0.04 arcsec pixels. Using the the CRTS data in a forthcoming paper. HST resolution limit and the distance to CSS100217 based on the redshift, we find that CSS100217 occurred within ∼150 pc We thank Minjin Kim for help in analyzing the SDSS spec- of the galactic nucleus. trum. The CRTS survey is supported by the U.S. National Sci- The probability of a chance alignment seems small, although ence Foundation under grants AST-0909182 and CNS-0540369. it cannot be rigorously excluded. Since NLS1 are expected to Support for program number GO proposal 12117 was provided largely occur in spiral galaxies (Crenshaw et al. 2003), with by NASA through a grant from the Space Telescope Science typical half-light radii of a few kpc, the possibility of a chance Institute, which is operated by the Association of Universities alignment along the line of sight but far from the AGN is a priori for Research in Astronomy, Inc., under NASA contract NAS very small, depending on the unknown central density profile 5-26555. The work at Caltech was supported in part by the of the host. We also note that there is no evidence for a redshift NASA Fermi grant 08-FERMI08-0025, and by the Ajax Foun- offset between the AGN emission and the event emission. Since dation. The CSS survey is funded by the National Aeronau- the narrow-line region in Seyfert 1 galaxies have been measured tics and Space Administration under grant no. NNG05GF22G to extend to sizes from 700 pc to 1.5 kpc (Bennert et al. 2006), issued through the Science Mission Directorate Near-Earth the event likely occurred well within the limits of the narrow-line Objects Observations Program. J.L.P. acknowledges support region. from NASA through Hubble Fellowship Grant HF-51261.01-A Significant star formation has been indicated within the awarded by the STScI, which is operated by AURA, Inc. for nuclear regions of number of Seyfert 1 galaxies and nuclear NASA, under contract NAS 5-26555. The PQ survey is sup- starbursts have been predicted in the dusty tori of Seyfert ported by the U.S. National Science Foundation under Grants galaxies (Imanishi & Wada 2004;Riffeletal.2007). The AST-0407448 and AST-0407297. Support for M.C. is provided presence of rapid star formation would naturally lead to Type II by Proyecto Basal PFB-06/2007, by FONDAP Centro de As- supernovae in such regions. However, there is no morphological trof´ısica 15010003, and by MIDEPLAN Rs Programa Iniciativa evidence of star-forming regions or superposed dust lanes Cient´ıfica Milenio through grant P07-021-F, awarded to The outside of the nucleus, and again, the broad range of wavelengths Milky Way Millennium Nucleus. GALEX (Galaxy Evolution (U to K bands) as well as the lack of any reddening signature Explorer) is a NASA Small Explorer, launched in 2003 April. in our spectra argue against it being hidden by dust. This We gratefully acknowledge NASA’s support for construction, suggests that any star-forming activity is likely within the region operation, and science analysis for the GALEX mission, devel- dominated by the AGN, rather than in some unrelated region in oped in cooperation with the Centre National d’Etudes Spatiales its vicinity. of France and the Korean Ministry of Science and Technology. On the other hand, a strong UVX radiation field in the The Expanded Very Large Array is operated by the National Ra- vicinity of the AGN would preclude star formation, unless it dio Astronomy Observatory, a facility of the National Science is well shielded. One interesting possibility is that the event is Foundation operated under cooperative agreement by Associ- a supernova associated with the outer edges of the accretion ated Universities, Inc. We thank the staff of GMRT that made disk, where it would be shielded from the AGN radiation. these observations possible. GMRT is run by the National Cen- These outer regions of the accretion disks are expected to be tre for Radio Astrophysics of the Tata Institute of Fundamental violently unstable, naturally leading to fragmentation and star Research. The Fermi LAT Collaboration acknowledges support formation, as predicted originally by Shlosman & Begelman from a number of agencies and institutes for both the devel- (1987, 1989), and further fortified by the modern numerical opment and the operation of the LAT as well as scientific data and semi-analytical models (Goodman 2003; Goodman & Tan analysis. These include NASA and DOE in the United States, 2004; Jiang & Goodman 2011). Moreover, these models favor CEA/Irfu and IN2P3/CNRS in France, ASI and INFN in Italy, the formation of very massive stars, which would fit naturally MEXT, KEK, and JAXA in Japan, and the K. A. Wallenberg with the event as a hyperluminous supernova. 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